The field of the invention relates to air/fuel ratio control of the mixture of air and fuel inducted into an internal combustion engine.
It is desired to maintain the air/fuel ratio at a predetermined or desired level within the operating window of conventional three-way (HC, CO, NO.sub.x) catalytic converters. Typically, conventional catalytic converters operate most efficiently when the air/fuel ratio is near 14.17 lbs. air/1 lb. fuel, a condition referred as stoichiometry.
Exhaust gas oxygen sensors (EGO) are commonly employed in feedback loops to regulate the air/fuel ratio about stoichiometry. Since the EGO sensor voltage output is only proportional to actual air/fuel ratio in a narrow region at stoichiometry, those prior approaches which have attempted to measure the actual air/fuel ratio directly from the EGO sensor voltage output have limited effectiveness. Although proportional exhaust gas oxygen sensors have been proposed wherein the sensor output is proportional to the actual air/furel ratio over a wide region around stoichiometry, these proportional devices are prohibitively expensive. Accordingly, typical approaches compare the conventional EGO sensor output to a reference associated with stoichiometry. A two-state switching device is thereby created for providing an indication of whether the air/fuel ratio is either on the rich side or the lean side of stoichiometry.
One known air/fuel ratio control system which employs such a two-state device is referred to as Ramp and Jumpback. In a typical example, the air/fuel mixture is gradually increased, or ramped, to the rich side until the EGO sensor detects a transition from a lean to a rich air/fuel ratio. The air/fuel ratio is then jumped to the lean side and ramped lean until the EGO sensor detects a transition from rich to lean. The process continues, ramping in alternating directions, resulting in an average excursion about stoichiometry.
The inventor herein has recognized that a problem with the above and similar approaches is that the air/fuel ratio continues to be ramped away from stoichiometry for a considerable time after the air/fuel mixture actually crosses stoichiometry. This is due to the time delay of an air/fuel charge through the intake manifold, engine, exhaust manifold, and EGO sensor. Besides causing emission and driveability problems, these excursions may saturate the EGO sensor further slowing the system response time.
U.S. Pat. No. 4,378,773 issued to Ohgami discloses a feedback control signal responsive to an EGO sensor wherein the air/fuel mixture is dithered to define three regions. More specifically, the air/fuel mixture is dithered six times per cycle such that there are two large rich excursions, one small rich excursion, two large lean excursions, and one small lean excursion. By counting the number of excursions detected by the EGO sensor, it may be determined whether stoichiometry is in one of the three regions. A disadvantage with this approach is that, apparently, operation is not centered or zeroed on stoichiometry.
U.S. Pat. No. 4,402,291 issued to Aono discloses another air/fuel ratio control system responsive to an exhaust gas oxygen sensor. The EGO sensor output is modulated with a bipolar signal to reduce the excursions in air/fuel ratios. A disadvantage of this system also appears to be that air/fuel operation is not zeroed on stoichiometry.